Memory metal actuators have been used in industry for some time and are described in the following patents:
______________________________________ U.S. PAT. NO. NAME OF INVENTOR ______________________________________ 5,317,612 to Bryan, et al 5,211,371 to Coffee 4,984,542 to Riche, et al 4,836,496 to Abujudom, et al 4,811,564 to Palmer 4,687,315 to Fujii 4,559,512 to Yaeger, et al 4,551,975 to Yamamoto, et al 4,523,605 to Ohkata 4,284,235 to Diermayer, et al 3,248,893 to McLane ______________________________________
The foregoing patents generally utilize memory metals in a known fashion, that is passing an electrical current through a shape memorizing spring causing the shape memorizing spring to heat up and expand thus creating movement within the actuator. The problem faced by many memory metal actuators is that for the current to pass through the shape memorizing spring, there must be an unbroken electrical current, a part of which includes the shape memorizing spring. The inclusion of the shape memorizing spring in the circuit creates difficulty when the shape memorizing spring expands as those portions of the circuit to which the shape memorizing spring is connected tend to move back and forth with the expansion and contraction of the shape memorizing spring. Such expansion and contraction of the shape memorizing spring tends to cause flexion, bending and twisting at the point where the shape memorizing spring is connected to other members forming the electrical circuit. Such other members can be wiring and/or the intended item being moved by the shape memorizing spring. It has been the experience of this inventor that the expansion and contraction of the shape memorizing spring will cause the connection between the shape memorizing spring and the other components of the circuit to crimp and break.
Another problem faced by memory metal actuators is that it is difficult to monitor the exact expansion of the shape memorizing spring relative to a specific electrical current passing therethrough. A spike of electrical current may cause the shape memorizing spring to hyper-extend causing damage to the shape memorizing spring and further overextending the movement of the member urged by the shape memorizing spring. In some applications, the actuator may be required to extend from a neutral to an optimal position at which point further extension is not necessary or is unwanted. It is not sufficient to simply block the expansion of the shape memorizing spring as the continual application of current to the shape memorizing spring in a fixed position, could overheat and damage the integrity of the spring. What is needed is an actuator using a shape memorizing spring, that will limit the movement of the shape memorizing spring without causing the shape memorizing spring to overheat.